Auto Vector Mask won't work

Sorry if this is a very naive question. I'm new to this forum and not long been using Fireworks. I have managed to create banners before though using this software, and I used the process of copying an image, flipping it vertical, then Commands > Creative > Auto Vector Mask and so forth so that I could create a reflection effect. This worked fine for my banners.

Now I'm trying to do the exact same thing but on a more detailed design than I previously worked on. I select the objects to reflect, but when I click on Auto Vector Mask I get a box pop-up saying 'Could not complete your request. The selection was incorrect for this operation.'

I'm not sure what I'm doing wrong, and why this worked before but not anymore.

I would be so grateful for any help, and again apologies for this beginner's question.